New gears are available from Randy's Ring and Pinion in WA. Runs about $200 a set. 3.23 gears were common in 60's slant cars, so you can look for one in the junkyard and swap the gears into your axle. You can estimate rpm change by using the percentage of change in your ratios.
7 1/4 axles aren't very strong, but they'll handle a mild six or even eight as long as they aren't abused with big tires/high stall converter/high rpm clutch drops/etc
. Smaller tires act like a fuse by breaking loose before something snaps in the rear.